Glass, as a material, has advantageous characteristics for many applications including: isotropy; considerable flexibility as to the shape and size of the finished glass objects; amenability to uniform doping at high concentrations; flexibility in achieving desired physical properties by virtue of the good solubility of the various glass composition constituents; and relatively low production costs.
Generally, glass is obtained by cooling a melt in such a way that crystallization is suppressed. Glass also can be produced by the known sol-gel method.
Most glasses are oxide glasses. The structure of oxide glasses consists of a continuous network of glass-forming oxides in which long range order is missing. Glass-forming oxides such as SiO.sub.2, P.sub.2 O.sub.5, GeO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3 and Ga.sub.2 O.sub.3 have the strongest bonding strength among glass-forming oxides. Such glass-forming oxides are known as glass network formers. Oxides with weak bonding strength, such as oxides of alkali, alkaline earth, and rare earths cannot form a glass network and are known as modifiers.
Some glasses are fluorescent. Fluorescent glasses, when exposed to ultraviolet light, convert that ultraviolet light into visible light. The fluorescence of rare earth metal ions in glass was first observed in the 1880s (see W. A. Weyl, "The Fluorescence of Glasses", in "Coloured Glasses", Society of Glass Technology, Sheffield, England, 1951). Fluorescent glasses are used in lasers, and the discovery of the lasing phenomenon gave a strong impetus to the development of rare earth activated fluorescent glasses. Various fluorescent glasses and their industrial applications are disclosed in U.S. Pat. Nos. 3,549,554; 3,846,142; 4,075,120 and 4,076,541.
Some glasses are photosensitive. When photosensitive glasses are irradiated with short wave radiation such as ultraviolet radiation or X-rays, the optical properties of the glass in the irradiated areas are modified. Photosensitive glasses generally contain photosensitive elements such as copper (Cu), silver (Ag) and gold (Au). The photosensitive elements in the glass, upon exposure to the incident radiation, absorb that radiation. Upon heat treatment of the glass (typically at or above the annealing point of the glass), metal particles are precipitated thus changing the color of the glass in the irradiated areas. Upon cooling of the glass, the colored areas remain colored unless the glass is subsequently reheated to a high temperature.
Photosensitivity was initially observed by Dalton and described in U.S. Pat. Nos. 2,326,012 and 2,422,472. Development of photosensitive glasses is described in U.S. Pat. No. 2,515,937.
While fluorescent glasses are known in the art, and while photosensitive glasses also are known in the art, it was not previously known to combine fluorescent properties with photosensitive properties in the same glass. Accordingly, it is desirable to provide a glass having both fluorescent and photosensitive properties.
It is also be desirable to be able to control the degree of fluorescence of the glass.
It further is desirable to be able to control the degree of fluorescence of the glass in selected areas of the glass.
It is also desirable to provide a glass having both fluorescent and photosensitive properties for use in photography and fluorescent displays.
It is desirable, moreover, to provide a glass in which the degree of fluorescence can be selectively controlled for use in computer memories.